Ignition timing spreadsheet
I think I've got it modeled pretty good. Attached you will find an Excel spreadsheet that models the three components of advance (static, mechanical and vacuum), adds them together and comes up with a timing curve for you. The "Specs" come from the manual.
Static: 13*
Vacuum: The book references 3 points, 20* @ 15" gauge Hg, 12* @ 10" gauge Hg, 3* @ 5" gauge Hg and I added the last one, 0* @ 0" gauge Hg since without vacuum, there is no vacuum advance. This is what had been tripping me up for a long time. The vacuum advance contribution is 20* worth anytime the kPa is at cruise or decel. Only when accelerating does it really drop off.
Mechanical: 1000 RPM = 0*, 2000 RPM = 10*, 4000 RPM = 20*
In the attached model, I upped the static to 15 and I got the mechanical all-in by 3000 instead of 4000 RPM. I also made an assumption that the 2 springs in the distributor are of different weight (typical) and that most of the advance comes early 15* of 20* for the first spring.
Variables you can play with:
Items in the light green can be changed to see the effect on the calculated table. These include the 2 springs' start and stop RPMs and the relative contribution of each, the amount of vacuum advance at different vacuum levels, the static advance and the maximum (safeguard) advance. You can also change the kPa and RPM scale to match yours and it *should* adjust accordingly. I put a ceiling in at 50* total advance, but it is ~53* (13* + 20* + 20*) mathematically. Call me conservative.
Idle "valley": Just like the stock setup, this table shows that I've set the timing to 15* right at idle and a little higher on either side of it. This creates a valley for the timing to settle into and really helps eliminating surging or hunting. I idle right around 775 RPM and 45 kPa once warm.
Engine Behavior: When you step on the gas and let more air into the engine, the load increases (move up the kPa scale) before the RPMs go up. As this happens, the RPM then starts to increase. So, if you are familiar with Megasquirt's little blue ball that indicates what's going on, it rises towards the top of the table before veering right. Immediately off idle it gets a lot of advance that is a function of the vacuum advance. As this decays when we approach 0* gauge (atmospheric), the mechanical advance is kicking in.
Anyway, download, play, experiment. I'm really pleased with my timing at this point and the power has improved dramatically. Don't forget to re-tune your fuel after changing your timing. Although I cannot detect any, predetonation (knock and ping) should be avoided at all cost. PLEASE BE CAREFUL AND USE AT YOUR OWN RISK. Compare my axis values to yours too.
OK, great, just went to attach file and it wouldn't let me upload. Mods, is there a problem with .xlsx files? Anyone that wants this, I'll have to email it to you in the meantime......
